1. Field of the Invention
This invention relates to electromagnetic geophysical mapping instruments.
2. Review of the Art
In my co-pending patent application Ser. No. 08/657,454 now U.S. Pat. No. 5,796,253 the disclosure and drawings of which are incorporated herewith by reference, there is disclosed a time domain electromagnetic geophysical mapping instrument having a transmitter, the transmitter having a transmitter including a transmitter coil, the transmitter generating intermittent current pulse waveforms, formed by half-sinusoidal output wave form segments of a waveform having a frequency equal to a resonant frequency of the transmitter coil, and a receiver including a receiver coil, in which the transmitter generates current pulses which typically comprise a plurality of immediately successive half-sinusoidal waveforms of common polarity.
Techniques for the application of frequency-domain electromagnetic (FDEM) instruments for general geological mapping and the direct detection of sub-surface metallic mineral deposits have been well documented over the years. FIGS. 1A and 1B depict typical FDEM system configuration for airborne and ground based systems respectively.
The technique is based on the measurement of the secondary magnetic field from subsurface targets, as a result of the primary alternating magnetic field established by the system transmitter.
In its simplest form, the typical system will operate at a single frequency with a single receiver transmitter coil pair. To obtain as much information as possible about the subsurface target a range of single frequencies is transmitted and the responses measured one at a time in sequence. To cover the necessary range of frequencies, a long measuring time is required. This makes system operation inefficient and time consuming.
Sometimes, especially in airborne systems, multiple transmitter and receiver sets are employed, operating at different frequencies, so that information at multiple frequencies is obtained simultaneously. The multi-coil system is very often complex, heavy and expensive to build and maintain.
A wideband current waveform can be generated to allow fast multi-frequency electromagnetic field measurement by employing pulse modulation techniques. The conventional pulse modulation technique utilizes a pseudo-random binary sequence, where the multi-frequency signal is generated in a simple, straight forward and controlled manner. The pseudo-random binary signal has well defined frequency spectrum and it is simple to build and implement.
FIG. 2A shows an example of a pseudo-random signal waveform a and FIG. 2B shows its power spectrum.
Other waveforms, for example a square waveform, can also be used to generate a spectrum of frequencies.
In order to achieve the maximum depth of exploration, having regard to external and internal system noise, it is necessary to maximize the transmitter dipole moment (product of transmitter coil area, number of turns and output current).
There is always constraint on transmitter coil area (size) and available power, so that a practical way to increase dipole moment is to increase the number of turns of the transmitter loop.
Since the inductance of a multi-turn loop is proportional to square of the number of turns according to the relationship L=KN2(a+b) where: L is inductance of the loop, N is number of turns, a and b are loop dimensions and K is a constant, multi-turns loop in general have large inductance and therefore high impedance that prevents the use of large currents. In order to overcome the high impedance of the transmitter loop, the output coil is often resonated with a capacitor to a particular frequency, a technique well known to the radio-engineering community.
If at a particular frequency, a capacitor in series with transmitter loop is selected so to have the same absolute impedance as the coil inductance, the maximum current will be supplied to the output coil. This is illustrated by FIG. 3, in which the circuit values shown are related as follows;             i      ⁡              (        t        )              =                  e        ⁡                  (          t          )                            r        +                  (                      ϖL            -                          j              ⁢                              1                ϖC                                              )                                        if              ❘              ϖL        ❘              =          xe2x80x83        ⁢                  ❘                              1            ϖC                    ❘                      
                    ⁢                                    Then                        ⁢                          xe2x80x83                        ⁢                          i              ⁡                              (                t                )                                                        ⁢              xe2x80x83            =              xe2x80x83            ⁢                        e          ⁡                      (            t            )                          R            
It is an object of the present invention to combine the efficiency of a tuned transmitter coil with a wide band output, using a transmitter arrangement of the kind described in the parent application; the invention employs such a transmitter in a frequency domain electromagnetic mapping instrument, utilizing for example a pseudo-random input signal waveform to the transmitter.
The invention thus provides a frequency domain electromagnetic geophysical mapping instrument comprising a transmitter and a transmitter coil connected to the transmitter, the transmitter generating current pulse waveforms, formed by half-sinusoidal output waveform segments of a waveform having a frequency equal to a resonant frequency of the transmitter coil, and a receiver, in which the transmitter generates current pulses comprising a plurality of immediately successive half-sinusoidal waveforms of common polarity, the pulses immediately succeeding each other being of alternating polarity. The pulse lengths may be equal, or vary in a manner providing a desired output spectrum, for example according to a pseudo-random sequence.